Add Documenter.jl documentation, migrated from the MLJ manual

[ablaom]

This documentation is to replaces two pages of the MLJ manual, one of which is very, very long. I don't do much extra here, apart from splitting the long page into a dozen or so separate pages for easier navigation.

Pages this PR replaces:

• https://alan-turing-institute.github.io/MLJ.jl/dev/quick_start_guide_to_adding_models/
• https://alan-turing-institute.github.io/MLJ.jl/dev/adding_models_for_general_use/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.53%. Comparing base (7578027) to head (306b824). Report is 10 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #191 +/- ## ======================================= Coverage 88.53% 88.53% ======================================= Files 8 8 Lines 375 375 ======================================= Hits 332 332 Misses 43 43 ```

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